1. Field of the Invention
The present invention relates to a sensor for sensing the surface displacement of an object to be inspected and more particularly a surface displacement sensor best adapted for use in focused position sensors of semiconductor device fabrication apparatus.
2. Related Background Art
So far as a focused point sensor in semiconductor device fabrication apparatus, there has been used an oblique incidence type focused position sensor of the type in which, as disclosed in detail in Laid-Open Japanese Patent Application No. 42205/1981, detection radiation is projected obliquely on the surface of a semiconductor wafer disposed at a position at which a mask pattern is transfered through a projection lens and in response to the reflected detection radiation, the positional displacement of the surface of the semiconductor with respect to a focused position is detected.
In the focused-position detection sensor of the type described, a slit-shaped radiation beam is projected on the surface of a semiconductor wafer which is to be detected and the reflected radiation beam is re-focused on a sensor including a photoelectric transducer and disposed in a photodetector or light receiving unit, whereby the position of the image re-focused on the sensor is detected.
When the surface of the semiconductor wafer which is to be detected is displaced vertically (in the direction of the optical axis of the projection lens), whether or not the surface of the wafer is located at the focused portion (reference position) with respect to the projection lens of the semiconductor fabrication apparatus based on the fact that in response to the vertical displacement of the surface of the wafer, the incident radiation beam on the sensor is caused to be displaced in a lateral direction in the direction perpendicular to the direction of incidence and then the lateral displacement is detected.
However, it was found out that the position detection accuracy is limited in the case of the detection of the position of the surface of the semiconductor wafer by the conventional oblique incidence type focused-position sensor of the type described above. In addition, it was also found out that when the re-focused image on the sensor by the reflected radiation beam from the surface to be detected is defocused, an error is introduced in the result of the position detection obtained by the sensor.